Does competition between soybeans and Wild Poinsettia with low-level resistance or susceptibility to glyphosate affect physiology and secondary metabolism?

The Wild Poinsettia (Euphorbia heterophylla L.) has elevated potential to reduce the soybean yield and is a difficult to control plant due to herbicide resistance. The objectives of this study were to determine the changes in the photosynthetic physiological parameters, secondary metabolite levels, cellular damage, and antioxidant system activity in soybean plants and Wild Poinsettia biotypes with a low level of resistance or susceptibility to glyphosate in competition with soybeans. Two replacement series experiments were conducted in a greenhouse under competition between soybeans and the low-level resistant biotype (experiment I) or susceptible biotype (experiment II). The treatments consisted of different proportions of soybean plants and Wild Poinsettia [100:0 (pure stand of soybean), 50:50, and 0:100 (pure stand of Wild Poinsettia)] and different evaluation periods (the 2013/14 and 2014/15 growing seasons). The following parameters were analyzed: the chlorophyll, carotenoid (CRT), total phenol, hydrogen peroxide (H2O2), and lipid peroxidation levels and the catalase (CAT), ascorbate peroxidase (APX), and superoxide dismutase (SOD) enzyme activity. Generally, competition between the soybeans and the Wild Poinsettia biotypes did not alter the photosynthetic parameters or the photosynthetic pigment levels of the crop. For the weed biotypes, competition with the soybeans negatively affected photosynthesis. Greater losses of chlorophylls and CRTs were observed for the susceptible biotype when in competition with soybeans, whereas the opposite trend was observed for the biotype with a low level of resistance. No cellular damage or alterations in antioxidant system enzyme activity was observed, which indicated a lack of oxidative stress.